Reducing energy consumption is important in the development and improvement of electronic devices, in particular if they are mobile or portable electronic devices. In order to save energy, electronic devices are more and more controlled by sophisticated schemes in which the magnitude of the consumed currents varies over several decades of magnitude. In low power modes some hundreds of nA (nano-amperes) of a current may be consumed while other operation modes require up to several hundreds of mA (milli-amperes). It is often necessary to measure these currents over a wide range (e.g. from nano-amperes to milliamperes) with an acceptable accuracy while at the same time being able to track highly dynamic current changes. Furthermore, any side effects due to measuring the consumed energy should be avoided or well controlled. For example, it is preferred that an increase of the energy consumption due to the energy measurement itself not occur.
One of the more common techniques for measuring a current is a measurement using a shunt device or a shunt resister. Using a shunt device for the power measurement requires very high precision analogue to digital converters in order to cover the full dynamic range of the possible magnitudes of the currents. For example, when four and a half decades of measurement with one percent precision is required, a 24-Bit-converter would be required. Furthermore, shunt devices generate a voltage drop. This voltage should be compensated, while the compensation circuitry constitutes a potential source of errors. Direct load compensation can be difficult. This means that the measurement range and therefore the circuitry used for measuring the power consumption has to be adapted during the energy measurement procedure. This increases complexity and entails more potential errors.
Still further, measuring a current indirectly by measuring the voltage across a shunt device requires an initial voltage change on the target. If a buffer capacitor is coupled to the target side (output side of an energy transfer circuits), the buffer capacitor delivers current immediately and needs to be recharged. This behavior affects the true current response of the device under test. Another approach of measuring the energy consumption employs a current mirror. One side of the current mirror delivers the current to the target including the target capacitor. The other side of the current mirror is coupled to an Ampere meter to which the mirrored current is fed. This approach has the advantage that the distortion caused by the target capacitor is minimized. However, the required pairing of the power and sense field effect transistors (FET) is rather poor and is not capable of tracking the huge current magnitude to be supported.